for the most part I would have thought the idea was mostly to keep delivery and passenger services out of the traffic lanes

I'm sure this varies from one locale to another, but in my experience in various parts of the US, virtually all urban roads with extra width have it just for parking. Much of the time the parking is so full that there's no room for delivery vehicles and passenger dropoff/pickup to use anything but the travel lanes, and even when there is room they often can't be bothered.

There are places where space is carved out just for loading and not parking, but it's not the common thing, especially in residential areas.

As a property owner will tell you. That public land isnt public but is an easement on private land.

I thought it was the other way around, typically: the city owns not only the street but a fair distance beyond the curb, and the adjoining property owner is required to maintain that area even though it's not theirs.